The late governor of West Darfur state Khamis Abdullah Abakar, in an unknown location in Sudan. AFP
"All those responsible for this killing must be held to account including those who bear command responsibility," Jeremy Laurence, spokesman for the UN rights office, told reporters in Geneva.
Since April 15, the regular army headed by Abdel Fattah al-Burhan has been locked in fighting with paramilitary Rapid Support Forces (RSF) commanded by his former deputy Mohamed Hamdan Daglo.
The death toll from two months of fighting has topped 2,000, while the violence has driven 2.2 million people from their homes, including 528,000 who have fled to neighbouring countries, according to the International Organization for Migration.
In long-troubled West Darfur state, Governor Khamis Abdullah Abakar was killed hours after he made remarks critical of the paramilitaries in a telephone interview with a Saudi TV channel on Wednesday.
UN rights chief Volker Turk "is appalled by the killing", Laurence said.
He pointed out that the killing on June 14 happened "just hours after he was arrested by the Rapid Support Forces in El Geneina capital of West Darfur, where the conflict has taken on an ethnic dimension."
"Alongside liability of the direct perpetrator, Governor Abakar was in the RSF custody, and it was RSF's responsibility to keep him safe," Laurence said.
He pointed out that the governor was the second high-profile person killed in El-Geneina in a matter of days, after the older brother of the traditional chief of the Masalit, Tariq Abdelrahman Bahreldin, was killed.
Laurence also voiced deep concern at rising hate speech in the region, warning "it could further inflame tensions."
The UN rights office, he said, had reviewed video clips recorded by members of Arab militia boasting about "victories" and killing and expelling members of other groups.
"We call for justice and accountability for extra judicial killings, and all other violations and abuses that have occurred during this ongoing conflict," Laurence said.